1. Field of the Invention
The present invention relates to a zoom lens for use in cameras and the like, and specifically relates to a zoom lens for varying the magnification at a plurality of predetermined magnifications.
2. Description of the Prior Art
The zoom lens used in cameras has a plurality of lens units, and has a zoom mechanism for arranging each lens unit at desired positions for zooming and has a focusing mechanism for arranging each lens unit at desired positions for focusing. A zoom mechanism capable of zooming at a plurality of magnifications and using the same mechanism for the focusing both simplifies construction and reduces the number of parts, and it also has the advantage of making the zoom lens more compact and reducing cost.
The construction of a conventional zoom lens for cameras having such a zoom mechanism is described below with reference to the brief section view shown in FIG. 1. As shown in the drawing, a helicoid 2a is formed on the interior surface of a stationary barrel 2 fixedly attached to the camera, and a helicoid 1a which meshes with the spiral of the helicoid 2a is formed on the exterior surface of a drive ring 1 which is rotatable within the stationary barrel 2.
A helicoid 1b is formed on the interior surface of the drive ring 1, and meshes with the spiral of a helicoid 3a provided on the exterior surface of a first moving frame 3 which supports a first lens unit L1, and the first moving frame 3 engages an advance guide groove 2b provided on the stationary barrel 2 via a connector 3b so as to be guided in advancement. A cam 1c is formed on the drive ring 1 and engages a guide pin 6 integratedly formed with a second moving frame 4 which supports a second lens unit L2, and the guide pin 6 advances in an advance guide groove 3c provided on the first moving frame 3.
When the drive ring 1 rotates, the drive ring 1 moves in the optical axis direction, that is, lateral direction in the drawing, via the helicoids 1a and 2a. The first lens unit L1 advances in extension via the helicoids 1b and 3a, and the second lens unit L2 advances in extension in accordance with the shape of the cam 1c. The amount of the extension of the first and the second lens units L1 and L2 is the combined amount of both the movement of the drive ring 1 via the helicoids 1a and 2a, and the movement of the first and the second moving frames 3 and 4 via the helicoids 1b and 3a and the cam 1c.
A brief development view of the drive ring 1 is shown in FIG. 2; the cam 1c comprises three identically shaped cams 20a, 20b, 20c, and three guide pins 6 are simultaneously connected. The helicoid 1b and the cam 1c are provided with retracting sections 10s and 20s to house the first and the second lens units L1 and L2 within the camera; when the camera operation is enabled, the first and the second lens units L1 and L2 are arranged near the end points e1 of the retracting sections 10s and 20s. At the end points e1, the helicoid 1b and the cam 1c are formed so as to set the first and the second lens units L1 and L2 to focus at infinity at a first magnification on the most wide angle side.
The zoom section of the helicoid 1b (not illustrated) and the zoom sections 22z, 23z, 24z, 25z of the cam 1c connect to the helicoid 3a and the guide pins 6 so as to move the first and the second lens units L1 and L2 and position the first and the second lens units L1 and L2 to focus at infinity at the second through fifth magnifications at the endpoints e2, e3, e4, e5 of the respective zoom sections.
When a desired magnification is selected and the first and the second lens units L1 and L2 are positioned near the endpoint of one zoom section, the distance to a photographic object is measured by a distance measuring device (not illustrated), and the first and the second lens units L1 and L2 are set at positions corresponding to the measured distance within the focus section of the helicoid 1b with the endpoint set as the starting point, and the focus sections 21f, 22f, 23f, 24f, 25f of the cam 1c to attain focus at the desired magnification.
According to the zoom lens of the aforesaid construction, the amount of movement of the first lens unit L1 relative to the rotational angle of the drive ring 1 is normally uniform in order that the movement of the first lens unit L1 is accomplished by the helicoids 1b and 3a. As a result, when the pressure angle .theta. of the helicoid 1b is reduced to reduce the torque and improve the resolution during focusing, an area of intersection (area D in the drawing) occurs between the helicoid 1b and the cam 20b. For this reason, the cam 20b is formed on the bottom part of the groove of the helicoid 1b to allow movement of the guide pin 6, thus increasing the thickness of the drive ring 1 and enlarging the zoom lens.
The endpoints (not illustrated) of the zoom sections at adjoining magnifications of the helicoid 1b cannot approach due to the focus section therebetween. The zoom magnification is restricted to the magnification corresponding to positions separated to some degree from each extension position of the first lens unit L1. Since the magnification changes particularly markedly with small movement of the first lens unit L1 at the wide angle side, camera operating characteristics are adversely affected inasmuch as the first through, e.g. fifth magnifications cannot be magnifications of uniform intervals.
When the amount of retraction of the lens barrel is increased to reduce the overall length of the zoom lens when housed, the retracting section 10s must be lengthened. Lengthening the retracting section 20s produces intersection of the cam 20a, 20b, 20c, and complicates the structure for normal connection of the guide pins 6 corresponding to the cams 20a, 20b, 20c, thereby enlarging the zoom lens.